In nearly all aspects of oil and gas production, pipelines operate at conditions suitable for the formation of hydrates. These hydrates can be quite stable and can be difficult to remove from lines which are plugged by such hydrates. Hydrates of light hydrocarbon gases typically form at temperatures and pressures in the neighborhood of about 40° F. and pressures of about 200 psia or greater. Note that seawater temperatures of 40° F. are relatively common in many oil and gas producing regions, as well as pressures an order of magnitude greater than 200 psia in the production systems. Particularly in the Gulf of Mexico, the formation of such hydrates has been a continuing problem with in-field lines, export pipelines, production wells, water injection wells, process piping and equipment.
There are basically four ways known to remediate such hydrate plugs. First, the pressure of the hydrate plug can be changed to a lower pressure, outside the stable range for the hydrates, thereby melting the hydrate. In many instances it may be difficult to lower the pressure below the hydrate stability pressure. In any event, when the pressure is lowered below the hydrate stability pressure, the decomposition of the hydrate is relatively slow, thereby requiring downtime in the production system for a substantial period of time to remove the hydrate. As well, if a large differential pressure is seen across the hydrate plug, it is likely for the plug to become a high speed projectile with the potential of causing equipment damage and/or personnel safety concern.
Secondly, the temperature of the hydrate can be increased above the hydrate stability temperature. As with the pressure technique described above, raising the temperature of the plug creates the potential for equipment damage and/or personnel safety concern.
Thirdly, the hydrates can be removed mechanically. While commonly used to remediate hydrate plugs in production wells, this method can be difficult to employ in production equipment and/or pipelines.
Fourthly, it is possible in some instances to inject a chemical such as alcohol or glycol to dissolve the hydrate. These liquids are effective in melting hydrates but are typically required in relatively large quantities if the plug is extensive. If the plug is a significant distance from the nearest injection location, this method may not be feasible.
It has long been known that hydrates exist in subsea environments and it has been hypothesized that these hydrates can be dissociated to become a source of methane gas production. A study directed to this technology is “Experimental Study on Dissociation Behavior of Methane Hydrate by Air” by Haneda H. et al. presented at the Fifth International conference on Gas Hydrates, Jun. 12-16, 2005, Trondheim, Norway. This reference is hereby incorporated in its entirety.
This reference deals with the recovery of methane from subsea hydrate deposits. It in no way suggests that the techniques disclosed therein could be used to remove hydrate plugs from pipe or production systems.
Accordingly, considerable effort has been directed toward the development of improved methods for removing hydrate plugs from pipe and production systems.